Electrical protective system



Dec. 29, 1936. L. K. SWART ,7

ELECTRICAL PROTECTIVE SYSTEM Filed Oct. 11, 1954 2 Sheets-Sheet 1 ZZoZREZag-s I? 5 INVENTOR l. Zi- Swami BY ATTORNEY Dec. 29, 1936. L. K. SWART 06 ,7

ELECTRICAL PROTECTIVE SYSTEM Filed Oct. 11, 1934 v 2 Sheets-Sheet 2 Ecantactqf'iCEelay 1 i L 0 C g? Z@'.6

g l L INVENTOR LZSwarZ ATTORNEY Ill Patented Dec. 29, 1936 UNETED STATES PATENT OFFICE Application October 11, 1934, Serial No. 7 17,93

14 Claims.

This invention relates to electrical protective systems. More particularly, this invention relates to arrangements for simultaneously ground ing a plurality of circuits when induced voltages above a fixed predetermined value become impressed thereon. Still more particularly, this invention relates to arrangements for testing the apparatus employed in carrying out the principles of this invention.

Pairs of protector blocks, each having an air gap which breaks down at a predetermined potential, have recently been used in the telephone art for the protection of each of a number of telephone circuits from high voltages set up therein by one or more sources extraneous to the circuits themselves. In these arrangements a resistor or saturating reactor is interposed in the ground connection of each pair of protector blocks or in the common ground connection of all the protector blocks. This resistor or reactor, as the case may be, is designed to carry all of the operating current flowing through all of the protector blocks associated therewith.

In some of these arrangements the impedance drop across the resistor or reactor is rectified by a rectifier which may be, for example, of the copper oxide type. The rectified current is applied to the winding of a relay which, when operated, shunts a pair or" the protector blocks. The rectified current is also supplied to a pilot relay which applies local battery to all the shortcircuiting relays which are associated with the apparatus for the purpose of grounding all of the protector blocks against the currents which traverse these blocks during the period in which extraneous voltages are applied to one or more of the lines.

In another protective arrangement the current traversing one or all of the protector blocks at a particular point along one or more exposed lines is applied to a saturating reactor or transformer. This current in turn is caused to operate an alternating current relay the contacts of which apply local battery to a plurality of shortcircuiting relays which are associated with the apparatus. These short-circuiting relays operate to short-circuit all of the protector blocks and thereby prevent these protector blocks from becoming permanently grounded.

The carbon protector block alone does not always operate within a narrow range of voltage. Accordingly, it is proposed that a gas discharge tube device be shunted around a protector block so that a fixed operating voltage depending upon the design and character of the gas discharge tube may be obtained. The shunt circuit around the protector block may include either the gas discharge tube device itself or a series circuit including in addition to that device a condenser, a relay, etc., as will be described more clearly here inafter. With apparatus or" the type just referred to the operating voltage may be made to lie above the normal telegraph and ringing voltages applied to the line or lines and below the minimum operating voltage of the carbon protector. Furthermore, the use of a gas discharge tube device, together with a condenser and relay, is practicable for operation of the protective apparatus from a voltage applied to the line from a remote point. The latter feature is particularl valuable for testing the apparatus connected with the line or lines from the remote point. In accordance with this invention a particular protective arrangement connected or coupled to the line may be selected, and a voltage or voltages of predetermined magnitude or magnitudes may be applied to the line to determine from a remote point the actual voltage at which a break-down may occur.

If a condenser is used in series with a gas dis- 2 charge tube arrangement such as just referred to, an alternating current potential maybe applied to the line from a remote point for testing the apparatus, and the magnitude of this alternating potential may, of course, be below that of the break-down voltage of the protector blocks associated with the line. This potential will produce a glow within the gas discharge tube and current will therefore be passed through the tube as well as through the associated relay. The operation of this relay may be employed to apply local battery to one or more short-circuiting relays in order that the line or lines with which the short-circuiting relays are associated may become temporarily grounded. While the line is thus momentarily grounded or short-circuited, a low potential may be applied, for example, to four of the conductors in parallel for the purpose of supplying sufiicient current to hold the relay protective device operated. This may be accomplished by passing current along these parallel conductors and through the contacts of the short-circuiting relay or relays and thence through the resistor or reactor or transformer to ground, thereby causing the equipment to hold up in the required manner.

This invention will be better understood from the detailed description hereinafter following when read in connection with the accompanying drawings, in "which Figure 1 shows a preferred cluding in a d ion to the gas discharge tub a form of rectiner; and Figs. 6, 7, S and 9 disclose other arrangements including various gas discharge tube devices in association with one or more exposed lines and in shunt with a protector block.

Referring to l of the drawings, the reference characters W1, W2, W3 and Wt represent four of a plurality of conductors or circuits or lines which may supported, for common protector pole, and each s ductors be connect-e: .o

an individual pair of prote common reactor designated blocks associated with the v ignated P1, P2, P3 and P4, L1 is shown as a winding h core, preferably this reactor may lorrn one of he elements of a transformer as shown. lower terminal of each of the protector bloc rs connected to the upper terminal or" the reactor L1, while the upper terminal of each block is connected to one of the conductors "5. 1 to W4. As well known, each of the various protector blocks may include two carbon electrodes spaced from each other by a gap of predetermined width which, for example, may be .003 inch.

The secondary winding L2 or" the transformer T is connected to the winding of an alternating current relay R0, the connection including a series condenser C1. The cond C1 is not a necessary element of the arrangement but is merely optional. If it is used it may act the circuit in series with the winding of relay R11 to some definite frequency, for examand R20 may be 5 sociated with the protector blocks P1 P4, in-

clusive. The windings of these relays nected in parallel relat onsh p,

circuit being arranged in the parallel and contact of the relay Re.

When the relay R0 becomes operated current will flow from the battery B through the arms. ture and contact of the relay R0 and through the windings of the short-chcuiting relays R and R211. The armatures of the latter relays will become attracted and the associated con tacts, which are preferably 0.1 the heavy d type, will provide paths shunt with the protee-tor blocks P1 to P4, inclusive. It will be apparent that the upper armature and contactof el Rio will shunt the protector block P1, lower armature and contact of the same .n shunt the protector block P2 and that and lower e and their rel of the protector blocks P1 to P4,

.lting these blocks the various con- W1 W 1, inclusive, will simultane- ""1 the reactor L1.

- uipmentalready described in connection with l, a conductor or line which l-litvclSC-S the entire range of the exposure and which is pretcrably grounded at the far end hrough some equipment such as a composite conductor. At a particular point along plot conductor, which may be r the purpose of ill" stration,

N2 be connected in ser'es with a condenser C2 and the winding of an alternating current relay RA, the connection ating at the high potential terminal of the c The armature and contact of the clay RA may be connected in multiple with the .rmature and contact of the relay R0, as shown. e armature and contact of the relay Ra will, a" 'o, perform the same function as "that ihe condenser C2 is not an indispensable eleof th s combination. It need be employed only if the service over the pilot conductor W1 involves the use of direct current with an associated path to ground. In tl arrangement the condenser erves to prevent e operation of the protective apparatus from e voltages employ-e l in the direct current service over that conductor.

The manner in which the apparatus of Fig. operates is as follows. Extraneous voltages may become induced in the various conductors W1, W2, W3 and W1. Taking the conductor W1 as an illustration and assuming that the voltage required to break down the space between the electrodes of the gas tube N2 is somewhat less than that required to break down the gap of the protector block P1, but greater than that required to cause the glow di charge between the electrodes of the gas tube N2, a discharge of substantial current will flow from the conductor W1 through the electrodes of the gas tube N, the condenser C2, the winding of the relay RA, the reactor L of transformer T and ground. The relay RA will become operated and the closure of its armature and contact will cause a flow of current from the battery B through the parallel windings of the R11 and Riv. armatures of these relays Pm and R will be simultaneously attracted, and therefore the conductors W1 to W4 will be connected to ground through the armatures and contacts of these relays and through the reactor L1, as already described. T-.is will allow more of the extraneous currents impressed upon the conductors W1, W2 and W3 and WA. to be passed through the reactor L. It will be apparent that if the extraneous voltages persist, the current through the reactor- L1 will be greater than that which formerly traversed it when the gas tube N2 alone provided the current which was transmitted through it. The increased current through the reactor L1 will be sufiicient to bring about the operation of the relay R0, and the relay R0 will remain operated as long as the current through the reactor L1 is suflicient for that purpose. As long as relay R11 remains operated all of the lines W1 to W. will be shunted to ground through the reactor L1.

The upper armature and contact of the relay R10, which short-circuits the protector block P1, also short-circuits the circuit which includes the gas tube N2, the condenser C2 and the winding of the relay RA. The gas tube N2 therefore remains operated only for a very brief interval of time, for example, a few thousandths of a second, which is the time required for the operation of the relay RA and the relays R10 and R20. It will be understood also that if the gas tube N2 is of sufficiently low impedance and is moreover capable of carrying sufficient current, it may, by discharge between its electrodes, furnish sufficient current through the reactor L1 of transformer T to operate the relay Re.

If it is desired to test the protective equipment shown in Fig. 1 from some remote point, an alternating current potential of a value below that required to break down the protector block P1 but in excess of that required to break down the space between the electrodes of the gas tube N2 and to overcome the impedance drop of the condenser C2 and winding of the relay RA, may be applied between the conductor W1 and ground at the remote testing point. This potential will immediately cause the operation of the relay RA. The closure of the contact of the relay RA will permit current from battery 13 toi flow through the parallel windings of relays R10 and R2, to operatethese relays, and the contacts of these relays will short-circuit the protector blocks P1 to Fr. At the same time the circuit of the gas tube N2 the condenser C2 and the winding of the relay RA will be shunted by the upper armature and contact of the relay R10, and therefore the impedance between the various conductors W1 to WA and ground will be relatively low. The application of a potential of but a few volts between any one or all of the conductors W1 to W4 and ground will cause suflicient current to flow through the reactor L1 of transformer T to operate the relay R0, and the latter relay will remain operated until the aforementioned low voltage has been withdrawn from these conductors.

In Fig. 2 the protector block P1 is also connected between the conductor W1 and the high potential side of the reactor L1 of the transformer T. In shunt with the protector block P1 there is a circuit which includes the primary winding of a transformer T1 and a condenser C2. The secondary winding of this transformer is connected in series with the gas tube N2 and the winding of the relay RA. As the voltage across the primary winding of the transformer T1 builds up to a value sufficient to impress a voltage across the electrodes of the gas tube N2 sufficient to break down the space therebetween, the relay RA will become operated. The closure of the contact of the relay RA may be employed, as in the arrangement of Fig. 1, to cause current to flow from a battery such as B, through the windings of short-circuiting relay such as R10 and R20, and the contacts of the latter relays may be employed to shunt protector blocks associated with other conductors (not shown in Fig. 2 but shown in Fig. l) It will be apparent also that the secondary winding of the transformer T1 may be connected to a relay winding such as R0, shown in Fig. l, the armature and contact of which will multiple the armature and contact of the relay RA.

In Fig. 3 a protector block P1 is associated with the conductor W1, but the circuit in shunt with the protector block P1 includes a pair of gas discharge tubes N21 and N22. These gas dis charge tubes may be of the rectifier type. They may be poled in mutual opposition, as shown, and they may be connected in parallel circuits in series with the two windings of the relay RA, as shown. The midpoint or terminal common to the windings of the relay RA may be connected to the high potential side of the reactor L1 of the transformer T.

If there is impressed between the conductor W1 and ground a voltage which is positive to the conductor and negative to ground and of sufficient magnitude, a current will pass from the conductor W1, through the gas tube N21 and through the left hand winding of the relay RA, through the reactor L1 and to ground. This current will be sufiicient to operate the relay RA even though no current flows through its other winding, since no current is allowed to flow therethrough by virtue of the poling of the gas tube rectifier N22. On the other hand, if a sufficient voltage is impressed between the con ductor L1 and ground which is positive to ground and negative to the conductor W1, a current will pass through the winding L1 of transformer T, right-hand winding of the relay RA and through the gas tube N22 and conductor W1. The current through the latter winding will opcrate the relay RA even though no current flows through the left-hand winding of that relay be cause of the poling of the gas tube rectifier N21. Thus the relay RA will be operated by voltage: produced by either half cycle of an alternating current voltage which may be applied between the conductor W1 and ground. Moreover, the relay RA may be operated by a direct potential of either polarity.

It will be understood that in the arrangement of Fig. 3 the devices N21 and N22 need not be of the gas tube type but may, if desired, be any form of rectifier well known in the art. It will be further understood that the secondary winding L2 of the transformer T may be connected to the winding of a relay such as R0, shown in Fig. 1, and that such an arrangement may be implied in the description of the other arrangements shown in the drawings of this case. It will be further understood that the armature and contact of the relay RA may be multipled to the armature and contact of the relay R0, as shown in Fig. l, and that this also may be considered as implied in the description of the other arrangements shown in the drawings of this case.

Fig. 4 shows an arrangement in which a plurality of gas discharge tubes N21, N22 and N23 are connected in series relationship in a circuit which shunts the protector block P1. These shunt circuits are interposed between the conductor L1 and the high potential side of the winding or reactor L1 of the transformer T. The tubes N21, N22 and N23 may, of course, be cold cathode gas filled tubes of any well known type.

The tubes N21, N22 and N22 are shunted by large resistances R1, R2 and R3 respectively. The employment of a multiplicity of seriesconnected gas filled tubes is particularly advantageous where it is desired to obtain operation at a volt well above the break-down voltage of a single tube alone. As a general rule the breakdown voltage of three series-connected tubes would be approximately equal to three times the break-down voltage of a single one of these tubes, if the break-down voltages of the various tubes are alike.

Howe if tubes are connected in series as shown in Fig. 4, it is necessary to shunt these tubes by the resistances R, R.- and R as shown, in order that the voltage applied across one of the tubes may substantially equal to that applied across the other or" the tubes. If the resistances R1, Pt: and H. were not employed, however, a particular gas tube of high leakage between electrodes would cause a voltage increase across the tube or tubes of lesser leakage, and the tube or tubes of lesser leakage would break down sooner than the tube or tubes of higher leakage. In fact, if the resistances R, R; and Hz were not employed, it would be virtually impossible to arrange that the break-down voltage of the series tubes would be equal to the sum of the break-down voltages of the individual tubes.

It will be apps nt, as before, that the winding L2 or" the transrormer T may be connected to circuits s lar to those shown in Fig. l, and will understood in the description of the other arangements hereinafter described. It will be further understood that while a relay such as RA is not shown in Fig. 4, such a relay may be included if desired, and its winding be located between the high potential side of the winding L1 and the right-hand terminal of the gas discharge tube N23, and th t the contacts of this relay may be arranged in circuits similar to those shown hereinabove and described in connection with Fig. 1.

In Fig. 5 no saturating reactor or transformer and no relay such as the one designated R0 are employed, but these are replaced by a resistor RR, a copper oxide rectifier arrangement Cc, copper oxide shunting devices E4 and i5 which are arranged in mutual opposition, a short-eircuiting relay Rm and a pilot relay Rm. This type of protective arrangement is understood in the art and its operation need not be further described. The apparatus of Fig. 5, however includes a gas discharge tube Nil, a relay RA and a condenser C: which are connected in a circuit in shunt with the protector block P1. The latter shunt circuit is arranged so that upon a discharge of current through the gas discharge tube N2; the relay EA will operate and in turn bring about the operation of the short-circuiting relay Rt, the contacts of which are employed to shunt the protector blocks P1 and P: out of the circuit.

If the gas discharge tube N24. is designed to operate at a lower voltage than the protector block P1, then upon the application of such. a voltage current may flow from the conductor Wl through the gas tube Nil, through the winding of the relay RA and the condenser 02 to the high potential side of the resistor RR, and thence through that resistor to ground. The armature 56 of the relay RA will close its contact ll. This will cause current to flow from the battery 13 over the circuit which includes the armature i8 and the contact ll of the relay RA, the normal release contacts of relay Rm, the winding of relay R30 and one arm of the copper oxide rectifier arrangement CU to ground. Upon the operation of the relay Run and the operation of similar relays the windings of which may be connected in sl unt with the winding of r Rm, all of the conductors such as W1 and W: will be connected to ground through the resistor RR.

It will be apparent fr 111 the description that the gas tube arrangements shown in Figs. 2, 3 and 4 may replace the gas tube shunt circuit shown in Fig. 5, and that in each case the tube arrangement will be connected in shunt with one of the protector blocks such as P], all of which is within the scope of this invention.

Fig. 6 shows an arrangement which is suitable for testing the circuit at a fixed voltage from a remote point. Here the conductor W1 acts as a pilot conductor, and each pair of conductors has a short-circuit relay (not shown) associated therewith and a protector block such as P1. The protector block P, shunted by a group of gas tubes N, N and N;; which are connected in series relationship, and these tubes are shunted by large resistances PM, and R respectively. The series arrange rent of tubes is also connected in series with the condenser C2 and the winding ill of the relay R0. The other side or" the winding is connected to the high potential side of the reactor L of the transformer T which extends to ground.

When current passes through the series discharge tubes N1, N2 nd it; through the cm denser C2 and the win lg 3 of the relay the relay R0 will become operated. This will bring about the closure of the contact of the relay R0 and will therefore complete a. circuit extending to the parallel connected windings short-circuitin relays via some source of current, such as a battery, and the operation of the latter relays will act to short-circuit the vicious protector blocks, only one of which is shown in this figure and is designated P1.

It is to be noted that the secondary win ing L; of the transformer T is directly connected to the winding IQ of the relay Re. A suflicient current impressed upon the winding by the transformer T will also operate the relay R0 and at the same time ring about the short-- circuiting oi the various protector blocks. It will be evident also that the contacts which are employed to shunt the protector blocks P1 and other similar blocks (not shown) likewise shunt the circuit which includes the three gas discharge tubes N1, N2 and Ni. It will be further apparent that when current is transmitted over the various discharge tubes N1, N2 and Ni through the circuit which includes the winding l8 of the relay R0, current will also be transmitted through the reactor L1 or primary winding of the transformer T, and this additional current will aid in maintaining the relay R0 continuously operated tl'iroughout the duration of the application of the extraneous voltage upon the conductor at W1.

In Fig. '7 the protector block Pi is shunted by a plurality of gas filled tubes designated Ni. These gas filled tubes are connected in series with individual resistances Ru, Pm. and If the voltage impressed between the conductor W1 and ground is of suiiicient magnitude to break down the gas discharge tubes N2 but of insuificient magnitude to break down the protector block P1, the operation the tubes N;- will allow suiiicient current to pass through the primary winding L1 of the transformer T, and this in turn will bring about the operation of the relay R0. Relay R0 will apply suihcient voltage to the windings of a lumber of short circuiting relays, which in turn will close the appropriate contacts for shunting the protector block P1 and additional blocks connected to other conductors similar to W1 but not shown. At the same time the circuits of the various gas discharge tubes N2 and their associated resistances R21, R25 and R26 will be shunted out of the circuit.

It will be apparent that the conductor W1 of Fig. '7 is employed to act as a pilot, and that the gas discharge tubes N2 are connected at a single protector point along this pilot conductor. At another point along a different conductor, such as W2, similar protector apparatus may be associated. At a third point along still another conductor W3 similar apparatus may be associated. A voltage sufficient to break down the various gas discharge tubes may be transmitted along each of the various conductors at a different time for selectively testing the apparatus connected to these various conductors from a remote point.

The relay R0 of Fig. 8 has two windings, 4| and 42. The winding M is connected to the reactor T. The winding 42 is connected in series with two groups of gas discharge tubes, as shown, each group including tubes N11 and N12. Gne of these groups of tubes is connected with the conductor W1 While the other group is connected with the conductor W2. The lower terminal of both groups of tubes is connected through the winding 42 of the relay R0 to the high potential side of the reactor T. I

When a potential exceeding a predetermined value is impressed between either conductor W1 or W2 and ground and is sufiicient to ionize the gas within the tubes N1 and N2 of either group, current will be passed through the winding 12 of the relay R0 and through the reactor T to ground. This current will operate the relay R0 and its contacts will short-circuit the protector blocks P1 and P2 as well as the groups of tubes N11 and N12 associated with these protector blocks. The operation of the relay R0 reduces the impedance between the conductors W1 and W2 on the high potential side of the reactor T to a negligible value and this, in effect, tends to increase the current through the reactor T. The increased potential set up across the reactor T will hold the armatures of the relay R0 closed throughout the duration of the extraneous potential. The relay R0 will release upon the removal or disappearance of the induced extraneous potential on the conductors W1 and W2.

It will be noted that resistances R1 and R2 are connected in shunt with the discharge devices N11 and N12 respectively. These resistances are employed for the purpose of maintaining the discharge characteristics of both devices substantially the same.

In Fig. 9 there is shown an arrangement which does not require any protector blocks. The conductors W1 and W2 are protected by the armatures and contacts of the short-circuiting relay R0.

Both of the conductors W1 and W2 are connected to the high potential side of the winding of the relay R0 through condenser C in series with the gas discharge devices N11 and N12, the latter being shunted respectively by resistors R1 and R2. The low potential side of the relay Winding is connected to ground. It will be apparent that the condenser C may be eliminated if direct current potentials are not normally impressed upon the conductors W1 and W2.

If the potential impressed between the conductor W1 and ground is suificient to overcome the impedance of the relay winding and the condenser C after ionizing the gas filled tubes N11 and N12, a substantial current will flow through the tubes N11 and N12 and through the winding of the relay R0 and the condenser C. The relay Re Will then operate and its armatures will close their associated contacts. The lower armature of relay R0 and its contact will connect the conductor W1 to ground through the winding of the relay R0. The upper armature of the relay R11 and its contact will connect the conductor W2 to ground through the winding of the relay R9. Wh n the potential impressed between the conductors W1 and W2 and ground is reduced below the value just referred to, the relay R0 will release and the gas within the devices N11 and N12 will no longer remain ionized.

It will be apparent that each armature of the relay R0 and its contact when closed will shunt one of the groups of gas discharge devices N11 and N12. Consequently these devices will be effectively removed from the circuit immediately after the relay R0 is operated and will remain effectively removed from the circuit until the voltage across the winding of the relay R0 is insuflicient to maintain that relay operated.

will be apparent that the resistors R1 and R2 which shunt the devices N11 and N12 may be entirely dispensed with if a sufficient break down potential may be obtained from a single gas discharge device. These resistances R1. and R2 are employed only when two or more tubes are connected in series relationship. It will be also understood that the number of conductors such as W1 and W2 is not limited to two, as illustrated, but any additional number of such conductors may be similarly connected to the contacts of the relay R0 through appropriate gas discharge devices, and that the relay R0 may have a plurality of armatures with corresponding contacts to short-circuit all of the various gas discharge devices after the short interval of time necessary to operate the relay R0 has elapsed. This is all within the scope of this invention. It will be desirable, if a great many circuits are connected with a single relay winding, that the core of the relay be of the saturating type and sufliciently sensitive to operate upon the application of the discharge current obtained from a single conductor and yet be able to carry the heavy currents derived from a large number of conductors. This is also within the scope of this invention.

While this invention has been shown and described in certain particular arrangements merely for the purpose of illustration, it will be understood that the general principles of this invention may be applied to other and widely varied organizations without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is:

. 1. Protective apparatus for a current-carrying conductor exposed to inductive interference comprising a protector block, a reactor connected between ground and the current-carrying conductor through said protector block, and a filled tube bridged across said protector block by conductors which remain permanently closed.

2. Protective apparatus for a telephone line exposed to inductive interference comprising a protector block, a reactor connected between ground. and said line through said protector block, a gas filled tube, and means comprising a closed wire circuit interconnecting said gas filled tube and said protector block so that the tube will effectively shunt the protector block continuously during the application of an induced voltage between the line and ground which exceeds a predetermined value.

3. The combination of a conductor exposed. to inductive interference, a saturating reactor one terminal of which is grounded, a protector block connected between the ungrounded terminal of the reactor and conductor, a two-electrode gas filled tube, relay having its winding connected in a permanently closed circuit between the ungrounded terminal of the reactor through the gas filled tube and the conductor, and means responsive to the application of a predetermined voltage across the gas filled tube for operating said rxay and completely shorting the circuit between the conductor and the ungroun led terminal of the reactor.

The combination of a conductor exposed to inductive interference, two protect vc devices permanently connected operatively in parallel relationship between said conductor and ground, both devices having different predetermined break-down voltages, and means responsive to the application of a voltage exceeding the breakdown value or" the device or" lower break-clown voltage for completely shunting both devices out of the circuit only during the interval or" time through which said voltage remains applied.

5. The combination of a conductor, a protector block, a reactor connected in series with the protector block between the conductor and ground, a two-electrode gas filled tube, a condenser and a relay the winding of which is connected in series with said condenser and said gas filled tube in a circuit which shunts the protector block, and means r sponsive to the application of a voltage exceeding a predetermined value betwee" the conductor and ground for providing a path of negligible impedance around the protector block; said path comprising the circuit in shunt with said protector block.

6. The combination or" a plurali'y of conductors which are exposed to inductive interference, a saturating reactor, a plurality of protector blocks one associated with each or" said conductors, various conductors being connected to ground through the corresponding protector blocks and the saturating reactor, a two-electrode gas discharge device, a condenser, an alternating current relay the winding of which is connected in series with said condenser and said discharge device in a c'cuit which shunts one of said protector blocks, and means responsive to the o eration of the alternating current relay for completely shunting all or" the protector blocks and for lay-passing the inductive effects impressed upon the various conconductors through the saturating reactor.

'7. The combination of an exposed telephone line, a protector block connected between said line and ground, and a circuit permanently and operatively shunting said protector block which includes a two-electrode gas discharge tube having a lower break-down voltage than that of the protector block.

8. In combination with a telephone line exposed to inductive interference, a circuit including a protector block connected between said line and ground, a two-electrode gas discharge device permanently connected operatively in shunt with the protector block and having a break-down voltage which is lower than that of the protector block, and means responsive to a gaseous discharge through the gas filled tube for providing a circuit of low resistance across the protector block.

9. The combination of a line exposed to extraneous voltages, a first protection circuit interposed between the said line and ground, a second protection circuit in parallel with the first protection circuit, the second protection circuitincluding two oppositely poled rectifier-s, and means responsive to the flow of current through either rectifier to provide a complete shunting path for the first protection circuit.

10. The combination of a telephone line, a first protection circuit interposed between said line and ground, a second protection circuit in shunt with the first protection circuit, the second pro tection circuit including two oppositely poled rcctifiers and a relay having two windings which connected in series with the respective recti- Cers, and means responsive to the operation of said relay for providing a complete shunt around the first protection circuit.

cuits of high impedance connected between said line and ground, said parallel circuits respectively including oppositely poled rectifiers, and means responsive to the how of current through either rectifier to efiectively ground said line.

12. combination of a line, a first high impedance protection circuit connected between said line an ground, and a second high impedance protection circuit permanently connected operatively in parallel with the first protection circuit, the second protection circuit including a plurality of devices having discharge which are connected in series with each other.

3. The combination of a first protection circuit, a second protection circuit in parallel with the first protection circuit, second circuit including a plurality of two-electrode gas filled tubes connected in series relationship, and a plurality of elements of high impedance, each of said elements shunting one of said gas filled tubes.

14. A protection circuit for a telephone line comprising a plurality of two-electrode gas filled tubes which are connected in series with each other between said line and ground, and means for rendering the break-down voltage of said gas filled tubes substantially equal to the sum of the break-down voltages of the individual tubes, said means comprising a plurality of elements of high impedance, each of said elements shunting one of said tubes.

LELAND KASSON SWART.

l. The combination of a line, two parallel ci 

